Gold is generally recovered from ore by a cyanide leaching process. This procedure is used because of its simplicity and ability to work effectively at low ore concentrations. Cyanide however is toxic. Furthermore, it leaches relatively non-specifically causing other metals to dissolve into the leach solution providing a variety of metal cyanide complexes. These metal cyanide complexes can also be toxic. Leach solutions have been accidentally released from various process facilities causing environmental damage. Consequently, and especially over the last decade, alternatives to cyanide leaching have been sought.
Particular attention has been given to thiosulfate; but, recovery of gold from the thiosulfate leach solutions presents a problem. Activated carbon cannot be used to recover the gold like it is used with cyanide leach liquors because it has a very low affinity for gold thiosulfate. Further, in cyanide leach liquors, when copper is present in the ore, it competes for cyanide ions with gold and causes high cyanide-consumption rates. Because the copper in the ore is usually present in larger amounts than the gold, resulting copper-cyanide ions are also present at higher concentrations and thereby compete with gold-cyanide ions for the adsorption sites on the activated carbon (Jay, 2000). Thiosulfate, similarity, leaches both copper and gold. Yet, copper is needed to stabilize the thiosulfate as well as catalyze the leaching reaction.
Since carbon adsorption is so problematic, extensive research efforts have turned to developing resin adsorption for gold recovery. However, resins suffer from fouling and are inherently expensive. An anti-fouling and cheaper alternative is therefore needed to make thiosulfate technology competitive with cyanidation.
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